CLT is my home airport so I've been through immigration and customs at CLT many times and it has always been extremely quick. I am regularly out of the airport faster when coming in on an international flight than a domestic flight (they seem to be faster with the baggage coming off of the international flights). CLT security is also usually quite fast especially if you have pre-check or priority. There have occasionally been long security lines but usually if you head to another check point (such as E) you can find a much shorter line. Even considering that you may have to pay duty, I would not consider your new shorter connection time to be tight. I don't think you have anything to worry about and if it were me I'd leave the reservation alone.